Among the useful attributes of charge balanced voltage-to-frequency converters is the fact that they can be used to implement analog-to-digital converters. Charge balanced voltage-to-frequency converters generally include a capacitor, connected with an operational amplifier to form a current integrator, that is cyclically charged, first, in one direction and, second, in the opposite direction (i.e., charged and discharged). This is done at a frequency which changes linearly with the input voltage applied. The net charge applied to the capacitor during each cycle is zero, a result achieved by charging the capacitor in the first direction for a predetermined time. In response to the end of that time, the capacitor is charged in the second (opposite) direction until a predetermined level is reached. The rate at which the capacitor is charged in the first direction is controlled by the sum of a current derived from the input voltage and a fixed current source. The rate at which the capacitor is charged in the second direction is determined by a current derived from the input voltage. Therefore, the frequency of the charge and discharge cycles is a direct function of the input voltage magnitude.
Among the well known advantages of charge balanced voltage-to-frequency converters are: (1) the inherent filtering provided by the capacitor which is connected to and responsive to the input during the entire charge/discharge cycle, and (2) the fact that this eliminates the need for an anti-aliassing filter in systems where one is digitizing the input in discrete samples.
Several different techniques have been employed for enabling charge balanced voltage-to-frequency converters to handle bipolar input voltages. One prior art bipolar charge balanced voltage-to-frequency converter includes an absolute value circuit connected between the voltage source and the charge balanced converter circuit. In an idealized situation a converter with an absolute value circuit responds to input voltages of -10 volts, 0 volt and +10 volts to derive frequencies of (for example) 100 kHz, 10 kHz and 100 kHz, respectively The negative voltage levels are detected to control a polarity bit which is combined with the counted frequency, containing the magnitude information, to determine the complete reading. However, the absolute value circuit has a tendency to have slightly different gain factors for positive voltages relative to negative voltages. The different gain factors of the absolute value circuit introduce errors in the voltage versus frequency relationship of the converter so that, in the above example, the derived frequencies may be 100 kHz, 10 kHz and 99 kHz. Further, absolute value circuits with even the above inaccuracy generally require the use of high accuracy operational amplifiers which are slow to settle. The settling time required for a circuit combined with the preamplifier ahead of the voltage to frequency converter can easily be 100 microseconds. This could be shortened greatly by implementing the absolute value circuit after the preamplifier, but at the expense of added parts, hence added cost and error.
Another bipolar charge balanced voltage-to-frequency converter provides a bias to the converter, such that a zero voltage input is offset to a predetermined value at the input of the converter. The maximum negative and positive voltages applied to such converters result in the converter deriving minimum and maximum frequencies, respectively, while a zero input voltage results in the converter deriving a median output frequency. For example, the bias applied to such a converter causes the converter response to be 10 kHz, 55 kHz and 100 kHz, respectively, for input voltages of -10 volts, volt and +10 volts. Such converters have the advantage of simplicity over converters which use an absolute value circuit, but they sacrifice one bit of resolution when compared to unipolar converters or those which use an absolute value circuit. This lost bit of resolution means that such converters have only one-half the resolution that the other converters have. Thus, the accuracy of prior art converters using a built in bias is reduced considerably.
A third prior art bipolar charge balanced converter includes a pair of converters, one for positive voltages and a second for negative voltages. Such a construction is disadvantageous because of the doubled cost of two converters and the difficulties in obtaining two converters having exactly the same responses to voltages of the same amplitude but of opposite polarity.
It is, accordingly, an object of the present invention to provide a new and improved bipolar charge balanced voltage-to-frequency converter.
Another object of the invention is to provide a new and improved, high accuracy, high resolution charge balanced voltage-to-frequency converter.
Another object of the present invention is to provide a new and improved, relatively inexpensive and highly accurate charge balanced voltage-to-frequency converter.